The invention relates to a permanent cathode to be used as electrode in the electro-refining and/or recovery of metals, such as copper, zinc, cobalt or nickel.
The invention can be applied, for example, to the electro-refining of copper, wherein anode copper in the form of anodes is transferred onto cathodes by means of an electric current to provide cathode copper. The electro-refining of copper takes place is tanks, in which the anode copper and cathodes are placed by turns and which contain electrolytic liquid. The invention can also be applied, for example, to the electrolytic recovery of copper, nickel, cobalt or zinc.
At present, modern metal electrolyses mainly use what is called a permanent cathode technology, which is based on reducing a metal, such as copper, onto the surface of the mother plate of a permanent cathode made of a suitable steel grade. The metal in the form of a cathode metal half, such as a cathode copper half, is easy to strip from the surface of such a mother plate by a machine (stripping machine) that is built for the stripping. An advantage of the process over the conventional starter sheet technology includes the ability to recycle the permanent cathodes back to the process, and their good flatness (straightness).
The first permanent cathode plants employed what is called the ISA technology, wherein the detachability of the cathode metal was ensured by using a suitable wax both in connection with the edge strips on the sides of the mother plate and on the bottom edge of the mother plate. In the method, one permanent cathode always yields two separate cathode metal halves (both growth halves are separated, the weight being half of the conventional cathode metal). However, the wax used in the method may cause problems both in the electrolysis process and the quality of the cathode metal. Some also consider the light weight of the cathode metal halves to be a problem, as it influences the casting capacity of the foundry in foundries, where the cathodes are fed into the melting furnace one by one.
Another prevailing permanent technology used is the so-called Kidd process, wherein the waxing of the bottom edge of the mother plate of the permanent cathode is omitted and the cathode metal halves are allowed to grow together at their bottom edges, resulting in what is called a taco cathode. If the bottom edge of the permanent cathode plate is completely flat, problems may arise when stripping the metal, as the metal partly gets stuck at the bottom edge of the mother plate. Due to this, the cathode metals thus obtained may have to be pressed straight or straightened in another way, since in stripping, the lower parts of the cathode metal halves curve to some extent, forming a crease/bag.
Both technologies have further been improved by cutting a V groove on the bottom edge of the permanent cathode mother plate. When a suitably deep V groove is used in the ISA technology, the cathode metal halves break off from each other at their bottom edges without waxing. In the Kidd technology, the V groove furthers the stripping of the cathode metal but may cause the cathode metal halves to break off from each other. In that case, some metal cathodes are of the taco style and some are of the ISA style. This in turn may be problematic for the user of the cathode.
In addition to the depth and shape of the groove, the running parameters used in the electrolysis also influence the detachment of the cathode metal halves from each other, when producing taco cathodes. These include, among others, the composition of the electrolyte, e.g., the additives and temperature, the mutual dimensions of the anodes and the cathodes, and their distance from each other and the current density used. Consequently, optimizing the depth and the shape of the groove may be quite challenging, as different electrolytic plants have their own preferences regarding the running parameters of the process.
Publication U.S. Pat. No. 3,798,151 presents a permanent cathode plate.
Publication WO 2004/097076 presents a permanent cathode plate.